BACKGROUND: The dilute Russell's viper venom time (DRVVT) test is part of the diagnostic armamentarium used to detect lupus anticoagulant (LA). When testing patients on warfarin therapy, there is some concern of false positive results due to their low Factor X levels. We studied the diagnostic performance of the DRVVT ratio (DRVVT-R) to confirm the presence of LA in thrombophilia patients receiving warfarin therapy, and compared those results with a control group receiving warfarin for cardiac conditions but without thrombosis. METHODS: The DRVVT (screen, confirm, and ratio), Factors II and X assays, and PT/INR were performed in patients receiving warfarin for thrombosis and in patients with cardiac conditions but no thrombosis (control group). RESULTS: Patients on warfarin in the thrombosis group (n=22) were positive for LA by DRVVT-R (ratio >1.27 was considered positive) whereas none of the patients in the control group (n=13) were positive for LA. The median DRVVT-R was significantly higher in the thrombosis group (1.60, range 1.29-1.92) as compared to controls (1.13, range 0.79-1.23, p<0.001) even though the INRs were comparable (median 2.3 for thrombosis group versus median of 2.2 for controls, p<0.05). Similarly, FX and FII levels were not significantly different in these two groups. CONCLUSIONS: We conclude that the use of the DRVVT-R allows for diagnosis of LA in patients receiving warfarin with therapeutic INR despite their decreased Factor X levels.